System and Method for Orthogonal Frequency Division Multiple Access

ABSTRACT

An embodiment OFDMA frame includes a header with signal (SIG) fields that are encoded at different sampling rates than one another. In one example, a first signal field is encoded at a 64 point fast frequency transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate. The first SIG field may carry parameters for decoding the second SIG field, and the second SIG field may carry resource allocation information for a payload of the OFDMA frame. The first SIG field may carry an identifier of an access point that transmits the OFDMA frame.

This patent application claims priority to U.S. Provisional Patent Application No. 62/011,475, filed on Jun. 12, 2014 and entitled “System and Method for OFDMA Tone Allocation in Next Generation Wi-Fi Networks,” to U.S. Provisional Application No. 62/020,902, filed on Jul. 3, 2014 and entitled “System and Method for Orthogonal Frequency Division Multiple Access,” and to U.S. Provisional Application No. 62/028,208, filed on Jul. 23, 2014 and entitled “System and Method for OFDMA Resource Allocation,” each of which are hereby incorporated by reference herein as if reproduced in its entirety.

TECHNICAL FIELD

The present invention relates to a system and method for wireless communications, and, in particular embodiments, to a system and method for orthogonal frequency division multiple access (OFDMA).

BACKGROUND

Next generation Wireless Local Area Networks (WLANs) will be deployed in high-density environments that include multiple access points providing wireless access to large numbers of mobile stations in the same geographical area. Next-generation WLANs will also need to simultaneously support various traffic types having diverse quality of service (QoS) requirements, as mobile devices are increasingly used to access streaming video, mobile gaming, and other services. Institute of Electrical and Electronics Engineers (IEEE) 802.11ax is being developed to address these challenges, and is expected to provide up to four times the throughput of IEEE 802.11ac networks.

SUMMARY OF THE INVENTION

Technical advantages are generally achieved, by embodiments of this disclosure which describe system and method for orthogonal frequency division multiple access.

In accordance with an embodiment, a method for transmitting data in a wireless network is provided. In this example, the method includes transmitting an orthogonal frequency division multiple access (OFDMA) frame to one or more mobile devices over a 20 megahertz (MHz) frequency channel. The OFDMA frame comprises a frame header that includes a first signal (SIG) field and a second SIG field. The first SIG field is encoded at a different sampling rate than the second SIG field. An apparatus for performing this method is also provided.

In accordance with another embodiment, a method for receiving data in a wireless network is provided. In this example, the method includes receiving an orthogonal frequency division multiple access (OFDMA) frame from an access point over a 20 megahertz (MHz) frequency channel. The OFDMA frame comprising a frame header that includes a first signal (SIG) field and a second SIG field. The first SIG field is encoded at a different sampling rate than the second SIG field. The method further includes decoding the first SIG field to obtain parameters for decoding the second SIG field, and decoding the second SIG field in accordance with the parameters carried by the first SIG field to obtain scheduling information for a payload of the OFDMA frame. An apparatus for performing this method is also provided.

In accordance with yet another embodiment, a method for requesting uplink resource units in an Institute of Electrical and Electronics Engineers (IEEE) 802.11 network is provided. In this example, the method comprises transmitting a request frame in a contentious time window of an IEEE 802.11 channel. The request frame requests that uplink resources in a scheduled time window of the 802.11 channel be allocated to the STA. An apparatus for performing this method is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates a diagram of an embodiment wireless network;

FIG. 2 illustrates a diagram of an embodiment frame structure for a downlink OFDMA frame;

FIG. 3 illustrates a diagram of an embodiment tone allocation scheme for an orthogonal frequency division multiple access (OFDMA) frame;

FIG. 4 illustrates a diagram of an embodiment tone allocation scheme for a downlink OFDMA frame transmission;

FIG. 5 illustrates a diagram of an embodiment OFDMA frame format for an uplink transmission;

FIG. 6 illustrates a flowchart of an embodiment method for transmitting data in a wireless network;

FIG. 7 illustrates a flow chart of an embodiment method for an OFDMA frame transmission;

FIG. 8 illustrates a flow chart of an embodiment method for an OFDMA frame header transmission;

FIG. 9 illustrates a flow chart of an embodiment method for an uplink request frame transmission;

FIG. 10 illustrates a diagram of an embodiment processing system; and

FIG. 11 illustrates a diagram of an embodiment transceiver.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The structure, manufacture and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. OFDMA tone allocations are discussed in U.S. Non-Provisional application Ser. No. 14/738,411, which is incorporated by reference herein as if reproduced in its entirety.

Aspects of this disclosure communicate an embodiment OFDMA frame comprising a header with signal (SIG) fields that are encoded at different sampling rates than one another. In some embodiments, a first SIG field is encoded at sampling rate that is capable of being decoded by legacy mobile devices, while a second SIG field is encoded at sampling rate that is capable of being decoded by next-generation devices. In this way, the second SIG field may carry more information (per resource), while the first SIG field may allow the OFDMA frame to be decoded by legacy mobile devices. In one embodiment, the first signal field is encoded at a 64 point fast frequency transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate. In some embodiments, the first SIG field may carry parameters for decoding the second SIG field, and the second SIG field may carry resource allocation information for a payload of the OFDMA frame. The SIG fields may also carry identifiers associated with an access point and/or mobile stations. In some embodiments, a given FFT sampling rates refer to encoding a field at the sampling rate in a 20 MHz frequency channel. For instance, encoding a field at a 64 point FFT sampling rate may refer encoding the field at 64 FFT per 20 MHz frequency channel, while encoding a field at a 256 point FFT sampling rate may refer encoding the field at 256 FFT per 20 MHz frequency channel.

Aspects of this disclosure also provide an embodiment technique for requesting uplink resources. In an embodiment, an AP may periodically allocate a contentious time window of an IEEE 802.11 channel to STAs in the coverage area of the AP. Those STAs may request uplink resources by sending a request frame in the contentious time window. Each request frame may be transmitted using a code division multiple access (CDMA) code assigned to, or selected by, the co-responding STA, thereby avoiding collisions between request frames communicated over the same time-frequency resources by different STAs. This CDMA-based uplink transmission request scheme may allow the AP to isolate the request frames in the code domain. These and other details are described in greater detail below.

FIG. 1 illustrates a network 100 for communicating data. The network includes an access point (AP) 110 having a coverage area 101, mobile devices 120, as well as a backhaul network 130. The AP 110 may comprise any component capable of providing wireless access by, among other things, establishing uplink (dashed line) and/or downlink (dotted line) connections with the mobile devices 120, such as a base station, an enhanced base station (eNB), a femtocell, a WiFi Access Point and other wirelessly enabled devices. The mobile devices 120 may comprise any component capable of establishing a wireless connection with the AP, such as a mobile station (STA), a user equipment (UE), or other wirelessly enabled devices. The backhaul network 130 may be any component or collection of components that allow data to be exchanged between the AP 110 and a remote end. In some embodiments, there may be multiple such networks, and/or the network may comprise various other wireless devices, such as relays, low power nodes, etc.

FIG. 2 illustrates a diagram of an embodiment frame structure for a downlink (DL) OFDMA frame 200. As shown, the downlink OFDMA frame 200 comprises a legacy preamble field 202, a first signal (SIGA) field 204, and a second signal (SIGB) field 206, a preamble field 208, and a data payload field 210. In this example, the SIGA field 204 is encoded at a different sampling rate than the SIGB field 206. In some embodiments, the SIGB field 206 is encoded at a higher sampling rate than the SIG A field 204, which allows the SIGB field 206 to carry more information per resource. In one embodiment, the SIGA field 204 is encoded at a 64 point Fast Frequency Transform (FFT) sampling rate, and the SIGB field 206 is encoded at a 256 point FFT sampling rate. In such an embodiment, the SIGA field 204 may consist of 64 tones, and the SIGB field 206 may consist of 256 tones. In some embodiments, the SIGA field 204 includes parameters for decoding the SIGB field 206, and the SIGB field 206 includes resource allocation information for a mobile device. The resource allocation information may indicate which resources in the data payload field 210 have been allocated to carry data to the mobile device. The SIGA field 204 and/or the SIGB field 206 may also carry an identifier of the access point that transmitted the DL OFDMA frame 200. In an embodiment, one or both of the SIGA field 204 and the SIGB field 206 are referred to as TGax SIG fields.

The legacy preamble field 202 may be backward compatible with IEEE 802.11a/n networks. The legacy preamble field 202 may be used to synchronize the data payload field 210 and avoid interference with other neighboring STAs in a cell. In one embodiment, the legacy preamble field 202 and the SIGA field 204 are encoded at one sampling rate (e.g., a 64 point FFT sampling rate), while the SIGB field 206, the Preamble field 208, and the data payload field 210 are encoded at another sampling rate (e.g., a 256 point FFT sampling rate). In another embodiment, the legacy preamble field 202, the SIGA field 204, the SIGB field 206, and the Preamble field 208 may be encoded at 64 FFT per 20 MHz frequency channel, while the data payload field 210 may be encoded at 256 FFT per 20 MHz frequency channel. The Preamble field 208 may be used to synchronize the data payload field 210 and avoid interference with other neighboring STAs in the cell.

FIG. 3 is a diagram of an embodiment tone allocation scheme for a 256-tone payload 300 in an OFDMA frame communicated over a 20 MHz frequency channel. As shown, the 256-tone payload 300 includes two-hundred and thirty-four tones carried in RUs 310, and twenty-two tones 320 excluded from the RUs 310. The twenty-two tones 320 excluded from the RUs 310 may include null tones, pilot tones, reserved tones, or combinations thereof. Each of the RUs 310 carried in the 256-tone payload 300 consists of a multiple of 26 tones. In the example provided by FIG. 3, the two-hundred and thirty-four tones are distributed into nine RUs 310 such that each of the RUs consists of 26 tones (i.e., one multiple of 26 tones). However, it should be appreciated that the two-hundred and thirty-four tones may be distributed into fewer RUs. For example, the two-hundred and thirty-four tones may be distributed into three 78-tone RUs. It should also be appreciated that the two-hundred and thirty-four tones may be unevenly distributed into the RUs 310 such that at least two RUs in the 256-tone payload 300 are different sizes. In one example, the two-hundred and thirty-four tones are distributed into four 52-tone RUs and one 26-tone RU. In another example, the two-hundred and thirty-four tones are distributed into two 104-tone RUs and one 26-tone RU. In yet another example, all of the two-hundred and thirty-four tones are distributed into a single RU. Other configurations are also possible. It should also be appreciated that the twenty-two tones 320 excluded from the RUs 310 may be arranged in any location, or set of locations, within the 256-tone payload 300. For example, each of the twenty-two tones 320 could be positioned in a contiguous portion of the 256-tone payload 300, e.g., in the center, top, or bottom portion of the 256-tone payload. As another example, the twenty-two tones 320 could be distributed evenly, or unevenly, across the 256-tone payload 300, e.g., in-between the RUs 320, etc.

FIG. 4 illustrates a diagram of an embodiment tone allocation scheme for a 256-tone payload 400 in an OFDMA frame communicated over a 20 MHz frequency channel. As shown, the 256-tone payload 400 includes two-hundred and thirty-four tones carried in RUs 410, 8 common pilot tones 422, and 14 null tones 426. The common pilot tones 422 and the null tones 426 are excluded from the RUs 410. In one example, the 14 null tones 426 consist of 13 guard tones and 1 DC tone. In other examples, the 14 null tones 426 include multiple DC tones and 12 or fewer guard tones. Each of the RUs 410 consists of a multiple of 26 data tones. In one embodiment, the 256-tone payload 400 is carried in a downlink OFDMA frame.

FIG. 5 illustrates a diagram of another embodiment tone allocation scheme for a 256-tone payload 500 in an uplink OFDMA frame communicated over a 20 MHz frequency channel. As shown, the 256-tone payload 500 includes two-hundred and thirty-four tones carried in RUs 510, 8 reserved tones 522, and 14 null tones 526. The reserved tones 522 and the null tones 526 are excluded from the RUs 510. In one example, the 14 null tones 526 consist of 13 guard tones and 1 DC tone. In other examples, the 14 null tones 526 consist of multiple DC tones and fewer than 13 guard tones, e.g., 2 DC tones+12 guard tones, 3 DC tones+11 guard tones, etc. Each of the RUs 510 consists of a multiple of 26 tones, with each multiple of 26 tones consisting pilot tones and data tones. In the example configuration depicted by FIG. 5, each multiple of 26 tones in a given one of the RUs 510 consists of two pilot tones and twenty-four data tones (2 pilots+24 data tones). It should be appreciated that other configurations are also possible, e.g., 1 pilot+25 data tones, 3 pilots+23 data tones, etc. In the example configuration depicted by FIG. 5, the 8 reserved tones 522 are evenly distributed over the 256-tone payload 500. In such an example, the reserved tones 500 may serve as guard bands between RUs in the uplink OFDMA frame. It should be appreciated that the 8 reserved tones 522 may be distributed differently (e.g., unevenly) in the 256-tone payload 500, and that two or more of the reserved tones 522 may be positioned in a contiguous portion of the 256-tone payload 500. It should also be appreciated that the reserved tones 522 may be used for other purposes. In one embodiment, the 256-tone payload 500 is carried in an uplink OFDMA frame.

FIG. 7 illustrates a flow chart of an embodiment method 700 for OFDMA frame transmission, as might be performed by a transmitter. As shown, the method 700 begins at step 710, where the transmitter generates an OFDMA frame to be communicated over a 20 MHz frequency channel. The OFDMA frame comprises a 256-tone payload that includes 234 data tones, 8 pilot tones, and 14 null tones. The 14 null tones include guard tones and at least one DC tone. In one embodiment, the 14 null tones may include 11 guard tones and 3 DC tones. In another embodiment, the 14 null tones may include 13 guard tones and 1 DC tone. The 234 data tones may form 18 RUs each of which comprising 13 data tones. Subsequently, the method 700 proceeds to step 720, where the transmitter transmits the OFDMA frame to at least one receiver. These steps are generally carried out in sequence, however, under certain circumstances; there may be parallel aspects among the steps.

FIG. 8 illustrates a flow chart of an embodiment method 800 for OFDMA frame transmission, as may be performed by an access point (AP). As shown, the method 800 begins at step 810, where the AP generates an OFDMA frame for communicating over 20 MHz frequency channel. The OFDMA frame comprises a frame header and a payload. The OFDMA frame header comprises a legacy preamble, a first signaling (SIGA) field, a second signaling (SIGB) field, and a preamble field. In one embodiment, the SIGA field has a different number of tones than the SIGB field. For example, the SIGA field may consist of 64 tones, and the SIGB field may consist of 256 tones. In another embodiment, the SIGA field has the same number of tones as the SIGB field. The SIGA field may carry an identifier assigned to the AP, and parameters for decoding the SIGB field. The SIGB field may carry OFDMA resource allocation information for STAs in a cell. The SIGA field may exclude resource allocation information. Thereafter, the method 800 proceeds to step 820, where the AP transmits the OFDMA frame to one or more STAs in the cell. These steps are generally carried out in sequence, however, under certain circumstances; there may be parallel aspects among the steps.

FIG. 9 illustrates a flow chart of an embodiment method 900 for requesting uplink resources, as might be performed by a station (STA). As shown, the method 900 begins at step 910, where an STA identifies a contentious time window of an IEEE 802.11 channel. Subsequently, the method 900 proceeds to step 920, where the STA spreads a request frame in the contentious time window of the IEEE 802.11 channel. The request frame is requests that uplink resources in a scheduled time window of the 802.11 channel be allocated to the STA. In some embodiments, the request frame is transmitted using a code division multiple access (CDMA) code associated with the STA to avoid collisions with other STAs transmitting request frames to the AP. These steps are generally carried out in sequence, however, under certain circumstances; there may be parallel aspects among the steps.

In one embodiment, three RUs in an OFDMA frame tone allocation may be processed together for a channel encoding and an interleaving operation in order to accommodate a number of data (or coded) bits per RU in an integer number for the modulation and coding scheme 9 (MCS-9). In such an embodiment, the channel encoding and the interleaving operation may be achieved as the case of 80 MHz frequency channel defined in IEEE 802.11ac standard specifications. This allows backward compatibility for IEEE 802.11 networks performing RU basis encoding and interleaving operation. Table 1 shows an MCS level based on bit size per RU or a symbol for the case of 256 FFT per 20 MHz frequency channel.

TABLE 1 Number of Data Number of coded bits per RU or bits per RU of Symbol Symbol Bits (UL OFDMA/DL (UL OFDMA/DL Code per OFDMA/Dl-UL OFDMA/Dl-UL MCS QAM rate QAM OFDM) OFDM 0 BPSK 1/2 1 96/104/117 192/208/234 1 QPSK 1/2 2 192/208/234 384/416/468 2 QPSK 3/4 2 288/312/351 384/416/468 3 16QAM 1/2 4 384/416/468 768/832/936 4 16QAM 3/4 4 576/624/702 768/832/936 5 64QAM 2/3 6 768/832/936 1152/1248/1404 6 64QAM 3/4 6 864/936/1053 1152/1248/1404 7 64QAM 5/6 6 960/1040/1170 1152/1248/1404 8 256QAM 3/4 8 1152/1248/1404 1536/16641872 9 256QAM 5/6 8 1280/4160*/1560 1536/4992*/1872 (*represents bit per 3 symbols or 3RUs)

FIG. 10 illustrates a block diagram of an embodiment processing system 1000 for performing methods described herein, which may be installed in a host device. As shown, the processing system 1000 includes a processor 1004, a memory 1006, and interfaces 1010-1014, which may (or may not) be arranged as shown in FIG. 10. The processor 1004 may be any component or collection of components adapted to perform computations and/or other processing related tasks, and the memory 1006 may be any component or collection of components adapted to store programming and/or instructions for execution by the processor 1004. In an embodiment, the memory 1006 includes a non-transitory computer readable medium. The interfaces 1010, 1012, 1014 may be any component or collection of components that allow the processing system 1000 to communicate with other devices/components and/or a user. For example, one or more of the interfaces 1010, 1012, 1014 may be adapted to communicate data, control, or management messages from the processor 1004 to applications installed on the host device and/or a remote device. As another example, one or more of the interfaces 1010, 1012, 1014 may be adapted to allow a user or user device (e.g., personal computer (PC), etc.) to interact/communicate with the processing system 1000. The processing system 1000 may include additional components not depicted in FIG. 10, such as long term storage (e.g., non-volatile memory, etc.).

In some embodiments, the processing system 1000 is included in a network device that is accessing, or part otherwise of, a telecommunications network. In one example, the processing system 1000 is in a network-side device in a wireless or wireline telecommunications network, such as a base station, a relay station, a scheduler, a controller, a gateway, a router, an applications server, or any other device in the telecommunications network. In other embodiments, the processing system 1000 is in a user-side device accessing a wireless or wireline telecommunications network, such as a mobile station, a user equipment (UE), a personal computer (PC), a tablet, a wearable communications device (e.g., a smartwatch, etc.), or any other device adapted to access a telecommunications network.

In some embodiments, one or more of the interfaces 1010, 1012, 1014 connects the processing system 1000 to a transceiver adapted to transmit and receive signaling over the telecommunications network. FIG. 11 illustrates a block diagram of a transceiver 1100 adapted to transmit and receive signaling over a telecommunications network. The transceiver 1100 may be installed in a host device. As shown, the transceiver 1100 comprises a network-side interface 1102, a coupler 1104, a transmitter 1106, a receiver 1108, a signal processor 1110, and a device-side interface 1112. The network-side interface 1102 may include any component or collection of components adapted to transmit or receive signaling over a wireless or wireline telecommunications network. The coupler 1104 may include any component or collection of components adapted to facilitate bi-directional communication over the network-side interface 1102. The transmitter 1106 may include any component or collection of components (e.g., up-converter, power amplifier, etc.) adapted to convert a baseband signal into a modulated carrier signal suitable for transmission over the network-side interface 1102. The receiver 1108 may include any component or collection of components (e.g., down-converter, low noise amplifier, etc.) adapted to convert a carrier signal received over the network-side interface 1102 into a baseband signal. The signal processor 1110 may include any component or collection of components adapted to convert a baseband signal into a data signal suitable for communication over the device-side interface(s) 1112, or vice-versa. The device-side interface(s) 1112 may include any component or collection of components adapted to communicate data-signals between the signal processor 1110 and components within the host device (e.g., the processing system 600, local area network (LAN) ports, etc.).

The transceiver 1100 may transmit and receive signaling over any type of communications medium. In some embodiments, the transceiver 1100 transmits and receives signaling over a wireless medium. For example, the transceiver 1100 may be a wireless transceiver adapted to communicate in accordance with a wireless telecommunications protocol, such as a cellular protocol (e.g., long-term evolution (LTE), etc.), a wireless local area network (WLAN) protocol (e.g., Wi-Fi, etc.), or any other type of wireless protocol (e.g., Bluetooth, near field communication (NFC), etc.). In such embodiments, the network-side interface 1102 comprises one or more antenna/radiating elements. For example, the network-side interface 1102 may include a single antenna, multiple separate antennas, or a multi-antenna array configured for multi-layer communication, e.g., single input multiple output (SIMO), multiple input single output (MISO), multiple input multiple output (MIMO), etc. In other embodiments, the transceiver 1100 transmits and receives signaling over a wireline medium, e.g., twisted-pair cable, coaxial cable, optical fiber, etc. Specific processing systems and/or transceivers may utilize all of the components shown, or only a subset of the components, and levels of integration may vary from device to device.

The following references are related to subject matter of the present application. Each of these references is incorporated herein by reference in its entirety: [1] U.S. Provisional Patent Application Ser. No. 61/974,282, entitled “UL OFDMA Frame Format and Input/Output Configuration for IFFT module for OFDM(A) Numerologies,” filed Apr. 2, 2014; [2] U.S. Provisional Patent Application Ser. No. 62/001,394, entitled “System and Method for Utilizing Unused Tones in Tone-Interleaved Long Training Field,” filed May 21, 2014. While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A method for transmitting data in a wireless network, the method comprising: transmitting, by an access point, an orthogonal frequency division multiple access (OFDMA) frame to one or more mobile devices over a 20 megahertz (MHz) frequency channel, the OFDMA frame comprising a frame header that includes a first signal (SIG) field and a second SIG field, wherein the first SIG field is encoded at a different sampling rate than the second SIG field.
 2. The method of claim 1, wherein the first SIG field is encoded at a 64 point Fast Frequency Transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate.
 3. The method of claim 2, wherein the first SIG field consists of 64 tones, and the second SIG field consists of 256 tones.
 4. The method of claim 1, wherein the first SIG field carries an identifier assigned to the access point, and parameters for decoding the second SIG field, and wherein the second SIG field carries OFDMA resource allocation information for the destination mobile devices.
 5. The method of claim 4, wherein the first SIG field excludes resource allocation information.
 6. The method of claim 1, wherein the one or more mobile devices are WiFi-compliant mobile stations (STAs).
 7. An access point (AP) comprising: a processor; and a computer readable storage medium storing programming for execution by the processor, the programming including instructions to: transmit an orthogonal frequency division multiple access (OFDMA) frame to one or more mobile devices over a 20 megahertz (MHz) frequency channel, the OFDMA frame comprising a frame header that includes a first signal (SIG) field and a second SIG field, wherein the first SIG field is encoded at a different sampling rate than the second SIG field.
 8. The apparatus of claim 7, wherein the first SIG field is encoded at a 64 point Fast Frequency Transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate.
 9. The apparatus of claim 8, wherein the first SIG field consists of 64 tones, and the second SIG field consists of 256 tones.
 10. The apparatus of claim 7, wherein the first SIG field carries an identifier assigned to the access point, and parameters for decoding the second SIG field, and wherein the second SIG field carries OFDMA resource allocation information for the destination mobile devices.
 11. The apparatus of claim 7, wherein the first SIG field excludes resource allocation information.
 12. The apparatus of claim 7, wherein the one or more mobile devices are WiFi-compliant mobile stations (STAs).
 13. A method for receiving data in a wireless network, the method comprising: receiving, by a mobile device, an orthogonal frequency division multiple access (OFDMA) frame from an access point over a 20 megahertz (MHz) frequency channel, the OFDMA frame comprising a frame header that includes a first signal (SIG) field and a second SIG field, wherein the first SIG field is encoded at a different sampling rate than the second SIG field; decoding the first SIG field to obtain parameters for decoding the second SIG field; and decoding the second SIG field in accordance with the parameters carried by the first SIG field to obtain scheduling information for a payload of the OFDMA frame.
 14. The method of claim 13, wherein the first SIG field is encoded at a 64 point Fast Frequency Transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate.
 15. The method of claim 14, wherein the first SIG field consists of 64 tones, and the second SIG field consists of 256 tones.
 16. The method of claim 13, wherein the first SIG field excludes resource allocation information.
 17. The method of claim 1, wherein the mobile device is a WiFi-compliant mobile station (STA).
 18. A mobile device comprising: a processor; and a computer readable storage medium storing programming for execution by the processor, the programming including instructions to: receive an orthogonal frequency division multiple access (OFDMA) frame from an access point over a 20 megahertz (MHz) frequency channel, the OFDMA frame comprising a frame header that includes a first signal (SIG) field and a second SIG field, wherein the first SIG field is encoded at a different sampling rate than the second SIG field; decode the first SIG field to obtain parameters for decoding the second SIG field; and decode the second SIG field in accordance with the parameters carried by the first SIG field to obtain scheduling information for a payload of the OFDMA frame.
 19. The apparatus of claim 18, wherein the first SIG field is encoded at a 64 point Fast Frequency Transform (FFT) sampling rate, and the second SIG field is encoded at a 256 point FFT sampling rate.
 20. The apparatus of claim 19, wherein the first SIG field consists of 64 tones, and the second SIG field consists of 256 tones. 